Commercial aircraft commonly comprise an auxiliary power unit (also referred to as “APU” for “Auxiliary Power Unit”) that is frequently arranged in a tail cone of the aircraft, wherein this auxiliary power unit is designed on the basis of a turbo engine such as, for example, a turbojet engine and supplies the aircraft with compressed air and electric energy. In such an arrangement, the exhaust gas created during the operation is discharged into the surroundings of the aircraft through an exhaust gas outlet in the region of the tail cone such that none of the tail assemblies of the aircraft come in contact therewith, if possible under all conditions encountered while the aircraft is on the ground and in flight. Due to the frequent operation of the auxiliary power unit while the aircraft is on the ground, its noise emission may represent the nuisance for the airport and its surroundings. Silencers that are arranged on the exhaust gas outlet of the auxiliary power unit by means of a corresponding coupling and dampen the exhaust gas noise accordingly are used in order to reduce the noise emission on the ground.
It is known, for example, from EP 1 398 473 B1 and U.S. Pat. No. 6,772,857 B2 to reduce inflow noises on aircraft engines by means of sound-absorbing layers with a honeycomb structure and a reflector. Although a plurality of differently designed silencers for auxiliary power units are also known from the state of the art, one specific design is given special consideration below.
In this design, a flow channel is provided that connects an exhaust gas inlet to an exhaust gas outlet and is surrounded by a housing. The volume between the housing and the flow channel is divided into several cells by means of intermediate walls (also referred to as partitions), wherein the majority of the partitions is equidistantly arranged along the flow channel. The flow channel is usually composed of a metallic, felt-like material that withstands the temperature and corrosiveness of the exhaust gas and is also porous. This not only makes it possible to route the exhaust gas through the flow channel along its extension in the housing, but also allows an alternating flow transverse thereto through the wall material of the flow channel and into the individual cells such that friction converts sound energy into heat during the passage through the wall material of the flow channel and the exhaust gas noise therefore is reduced.
One problem of known silencers of this design is, for example, an insufficient ratio between the diameter of the flow channel and of the housing such that, for example, the outer diameter of the cells is relatively low by comparison and, in particular, a first occurring transversal mode of the exhaust gas sound therefore can only be weakly dampened. In addition, longer cells are required in order to considerably dampen all occurring transversal modes such that the sound damping efficiency per length unit of the silencer ultimately drops. However, there are also known silencers, in which the diameter of the housing is sufficient such that the first transversal mode can be dampened sufficiently strong, but not the second transversal mode. In broadband noise, the sound energy is predominantly transmitted in the range of resonant frequencies due to the generation of resonant modes in a silencer. Another disadvantage of these known silencers consequently can be seen in that the sound dampening is very low for certain frequency ranges due to standing waves in the silencer.
In the design of a corresponding silencer, the available installation space is very restricted, particularly in aircraft, such that the external dimensions of the silencer cannot be arbitrarily enlarged in order to increase the degree of sound dampening, but rather should always be maintained constant or at least as small as possible.
In the existing designs of silencers for auxiliary power units of an aircraft, improvements with respect to the degree of sound dampening are not readily expected without enlarging the external dimensions of the silencer.